One of the great, untapped natural resources for creating renewable energy is from the tides. The world’s total ocean potential for tidal power is estimated to be 1,800TW/h per year, roughly sufficient to provide power for a country the size of the United States (of more than 300 million citizens). But to date very little of this immense resource has been captured - why?

One of the main reasons is cost, and concerns about possible effects on the marine environment - as seen from the House of Commons’ Energy and Climate Change Select Committee’s criticisms of the proposed Severn Barrage project last week.

But cost concerns too often stem from the fact that consideration is only given to short term costs. For example, the tidal barrage at La Rance in France was relatively expensive to build, and such capital outlay typically requires an investment of more than 30 years to cover. However, now completed and fully paid for, the tidal barrage has become one of the cheapest sources of electrical power anywhere in the world.

Tidal power can be acquired in two ways. One, by using the tides to drive turbines, extracting power directly from the tidal current. Or two, by extracting power from a barrage or lagoon where the water is held back at a higher level than on the other side of the barrage wall. The difference in height, known as head, generates pressure to drive turbines - a setup typically used in hydroelectric dams.

The amount of power generated depends on the intensity of the current or the head difference, both of which vary considerably with tide characteristics. But tidal energy has one very big advantage: unlike other forms of renewable energy, it is predictable. In fact, since the tides are created by the relative motion of the earth and moon around the sun, we can predict tides anywhere in the world well into the future. So tidal power has considerable potential and key advantages over other forms of renewable energy.

Artist’s impression of the proposed Severn Barrage.Hafren Power

As a still-developing industry, tidal power generating devices vary. The Marine Current Turbine (shown above at Strangford Lough) works on a horizontal axis, while others like the Gorlov helical turbine operate on a vertical axis, with the advantage of it being omni-directional (ie., independent of the direction of the current), as used widely in Korea.

Tidal barrages and lagoons are similar in concept but, whereas a barrage spans an estuary, a lagoon is either attached to the coast or can be sited offshore. Barrages can generate substantially more power than lagoons and provide flood defence, whereas lagoons are generally perceived to be less environmentally damaging. The main barrages built today are La Rance, in France, Annapolis Royal, in Canada, and Shiwa Lake, in Korea.

The Severn Barrage in the UK is one of the world’s largest proposed tidal barrage projects and has attracted considerable interest. Since 2008 extensive studies have been undertaken to establish the ideal site and project to maximise the power potential of the Severn Estuary, which has the world’s second largest tidal range of 14m during a mean spring tide. The findings showed that from an engineering perspective an 11 mile (16km) barrage reaching from southwest of Cardiff, Wales, to Brean Down, near Weston-super-Mare in Somerset, is the preferred option.

The proposed Severn Barrage would stretch 11 miles across the estuary.Roger Falconer

Hafren Power, previously Corlan Hafren, was formed to continue the work, and has proposed a new barrage design. If built, it will be one of the world’s largest generators of renewable energy, meeting around 5% of the UK’s electricity demand (16.5 TWh/yr). This is equivalent to three or four nuclear reactors or 3,000 wind turbines, would offset 7.1m tonnes of CO2 per year, and would provide considerable flood risk protection upstream.

The new Hafren Power design takes into consideration concerns raised. It operates in a two-way generation mode, producing power on both the incoming and ebb tide. As it no longer requires sluice gates to let the water back out, the number of turbines can be maximised - 1026 in all. It produces the same amount of power, but without disturbing the tidal form upstream, and leaving the groundwater level unchanged. It also brings substantial flood risk reduction benefits, not just in the estuary but some considerable distance upstream in the rivers Severn, Wye, Usk and Avon.

The barrage would use low-head, bi-directional turbines which, as they are spread out across the entire structure rather than clustered in the centre, would run at a lower velocity and be less damaging to fish. Design studies have been funded within Cardiff University’s Hydro-Environmental Research Centre through two EU grants.

“Hafren Power have failed to overcome the serious environmental concerns that have been raised. Further research, data and modelling are needed before environmental impacts can be determined with any certainty – in particular regarding fluvial flood risk.”

However, Hafren Power has always been fully aware that these studies are required for the environmental impact assessment. And I cannot help but wonder why are we not taking this opportunity to develop this vast resource around our coastline, when the barrage at La Rance in France now delivers Europe’s cheapest, most predictable, zero-carbon electricity.